The present invention relates to a compression system, a decompression system and compression/decompression system for compressing, decompressing and compressing/decompressing image data.
There is conventionally performed a three-dimensional orthogonal transformation (hereinafter refer as to a "DCT"--a discrete cosine transformation--corresponding to a necessity) in order to reduce redundancies in image signals, with respect to not only a direction-in-plane but also a time direction. In the three-dimensional DCTs, a DCT is performed with respect to pixels as an object which are the same positions as a two-dimension in an image plane and have discrepancies in time base.
Furthermore, there is conventionally provided a compression system for reducing redundancies in the time direction by combining a motion prediction and a two-dimension DCT, for example, a system which is used in a television conference/telephone system having "n.times.384 kb/sCODEC" and the like (refer to pp. 1219-1225, Vol. 42, No. 11, in "A trend of international standardization of a television conference/telephone system" of the bulletin of Japanese television society).
There will be described this system in accordance with a block diagram shown in FIG. 1. An outline of the system is that an in-plane discrete cosine transformer 4 codes a predictive error signal 1a capable of representing the difference between an input image signal aa and a predictive image signal 12a which is generated in accordance with a detected result of a motion vector.
In FIG. 1, the input image signal aa is supplied to a motion estimation means 11 to output to a motion prediction means 12 a motion vector signal 11a which is obtained by comparing the input image signal aa with a memory output signal 10a corresponding to the input image signal of a previous one frame. In this specification, one frame means one picture plane which is a concept including one field and one frame in interlaced scanning.
The motion prediction means 12 generates a predictive image signal 12a which is predicted from the memory output signal 10a on the basis of the motion vector signal 12a, and a subtracter 1 subtracts the predictive image signal 12a from the input image signal aa to obtain a predictive error signal la which is supplied to a prediction discrimination means 2 and a first selection means 3.
The prediction discrimination means 2 discriminates as to whether or not the prediction is effective by comparing an information amount of the input image signal aa with that of the prediction error signal 1a, namely, the means 2 discriminates that the prediction is effective when the information amount of the prediction error signal 1a is less than the information amount of the input image signal aa, and that the prediction is ineffective when the information amount of the prediction error signal 1a is more than the information amount of the input image signal aa. The first selection means 3 and a second selection means 9 operate on the basis of the discriminated result. Namely, the first selection means 3 selects the predictive error signal 1a when the prediction is effective, while the means 3 selects the input image signal aa when the prediction is ineffective, thereby supplying any of the signals 1a and aa to the discrete cosine transformer 4. The second selection means 9 selects the prediction image signal 12a when the prediction is effective, thereby supplying it to an addition means 8 described later.
A signal performed a two-dimension DCT by discrete cosine transformer 4, is quantized again by a quantization means 5 to supply to a coding means (not shown) an output image signal bb of which an information amount is compressed. The output image signal bb is decoded by an inverse quantization means 6 and an inverse discrete cosine transformer 7 constituting a local decoder to generate a signal corresponding to the prediction error signal 1a which is added by the addition means 8 with the prediction image signal 12a obtained through the second selection means 9, thereby obtaining a decoded signal 8a corresponding to the input image signal aa. The decoded signal 8a is supplied to a memory means 10 to obtain the memory output signal 10a which is delayed with one frame.
By this, in the compression system combining the motion prediction with the two-dimension DCT for reducing the redundancies in the time direction, the two dimension DCT is performed with the prediction error signal 1a, and requantization is performed to reduce the information amount. At the same time, by using the local decoder, the prediction image signal 12a is generated on the basis of the decoded signal 8a including quantization errors in order to avoid an accumulation of the quantization errors occurring in the requantization.
However, in the three-dimension DCT as described above, since the DCT is performed with pixels (as an object) which are positioned at the two-dimensional position on the picture plane and have discrepancies in the time base, when the object in the input image moves, the correlation in the time direction disappears, thereby resulting the problem that it is impossible to reduce the redundancies.
On the other hand, in the case where the motion prediction is combined with the two-dimension DCT, even though the object in the input image moves, it is possible to reduce the redundancies because the motion prediction means intensifies the correlation in the time direction. However, in this case, it is necessary to provide the local decoder, thereby resulting the problem that both of a calculation amount and a circuit scale become larger.